This invention relates to exhaust emission control systems employing a trap for catching carbon particulates in exhaust emissions of diesel engines, and, more particularly, to an exhaust emission control system capable of effectively regenerating the trap by subjecting the carbon particulates caught by the trap to oxidation treatment.
Depending upon load conditions, exhaust gases of diesel engines are generally exhausted in black fumes due largely to the presence of carbon particulates therein. An increase in motor vehicles has given rise to the problem of air pollution by the flue gases of combustion engines, and the black fumes exhausted by diesel engines are no exception. To avoid this problem, an exhaust emission control system using a trap for catching carbon particulates has been in wide use.
In this type of exhaust emission control system, the carbon particulates caught by the trap increases in amount during use, causing a blockage of the trap. Thus, it is necessary to regenerate the trap to catch carbon particulates by removing the particulate material adhering to the trap by subjecting the carbon particulates to oxidation treatment.
Several proposals have been made to solve the problem of blockage of the trap of an exhaust emission control system for a diesel engine.
More particularly, in for example, Japanese Patent Laid-Open No. 18016/81, an exhaust emission control system is proposed wherein a burner is located upstream of a trap formed as a ceramic filter of a honeycomb shape for catching carbon particulates. The back pressure of the engine is sensed when a predetermined amount of carbon particulates are trapped by the ceramic filter, to actuate the burner to burn the carbon particulates thereby removing the same from the ceramic filter.
In, for example, Japanese Patent Laid-Open No. 904112/81, another exhaust emission control system is proposed wherein a honey-comb shaped ceramic filter is provided with a coat of platinum serving as a catalyst. When a predetermined amount of carbon particulates are trapped by the ceramic filter, the back pressure of the engine is sensed and a fuel, such as propane, is fed through a fuel nozzle to the ceramic filter to activate the platinum catalyst. In all systems of the aforementioned type, the combustion gas from the burner or the fuel from the nozzle is directly introduced into the exhaust pipe. Thus, difficulties are experienced in creating the necessary conditions for oxidation treatment to enable a removing of the carbon particulates from the trap because such condition would be greatly influenced by changes in the physical and chemical properties of the exhaust emissions, thereby making it impossible to regenerate the trap effectively and efficiently.
In the exhaust emission control system proposed in aforementioned Japanese Patent Laid Open No. 18016/81, the trap is held at a high temperature of about 700.degree. C. to burn the carbon particulates. In this system, when the engine operation conditions vary and the pressure, flow rate, temperature and composition of the exhaust emission undergo changes, the temperature of the trap would also show a change, thereby making it impossible to continuously maintain the trap at a temperature at which the trap regeneration efficiency can be maximized.
In the exhaust emission control system proposed in aforementioned Japanese Patent Laid Open No. 904112/81, it is necessary to provide an atmosphere which is maintained substantially constant to ensure the existence of the conditions necessary for performing oxidation of the carbon particulates at a desired level. However, as with the system of Japanese Laid-Open No. 18016/81, this system also suffers the disadvantage that the atmosphere created can not be maintained substantially constant due to the changes occurring in the exhaust emissions, resulting in a reduction in regeneration efficiency.
This invention has been developed for the puspose of avoiding the disadvantages of the prior art. Accordingly, an aim of the present invention essentially resides in the provision of an exhaust emission control system for a diesel engine capable of creating a substantially constant condition for performing oxidation of the carbon particulates caught by the trap irrespective of changes occurring in exhaust emissions, thereby enabling regeneration of the trap to be performed with a high degree of efficiency at all times.
In accordance with advantageous features of the invention, an exhaust emission control system for a diesel engine is provided which comprises a trap in the form of a filter formed of heat-resisting material for trapping carbon particulates, with the trap being regenerated by subjecting the trapped carbon particulates to an oxidation treatment when the trap is blocked. The trap used is of a type which has no directionality with respect to air permeability, and a gas for heating the trap or a fuel for reactivating a catalyst is fed to the trap in a direction which is substantially perpendicular to a direction in which exhaust emissions flow from the diesel engine through the trap.